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OXIDATION AND AGGT,OMEMTION RESISTANCE OF THIN GAGE DISPERSION STRENGTHENED
ALLOYS
FINAL REPORT
D. M. Scruggs Department of Mechanical Engineering
University of Arkansas
30 June 1970
FOP The National Aeronautics h Space Administration
Grant NGR 04-001-029
FOREWORD
This work was carried out under the guidance of Mr. Bland Stein of
NASA Langley. His support and suggestions are gratefully acknowledged.
Two research assistants contributed i n great measure to th is work,
C. K, Moore and H. M. Foote. We are indebted to Mr. K. S . K i m for h i s
excellent e f forts with the Electron Microscope.
TABLE OF CONTENTS
FOREWORD
LIST OF TABLES
LIST OF FIGURES
SUNMARY
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
CONCLUSIONS AND RECOMMENDATIONS
APPENDIX
BIBLIOGRAPHY
Page
i
iii
V
1
2
6
8
29
31
33
35
ii
LIST OF TABLES
Table No.
I
I1
I11
IV
V
VI
VI1
VI11
IX
X
Title
Oxidation rates of an 80 Ni - 20 Cr alloy. Dispersion strengthened alloys examined in the present study.
Four alloys examined in the present study and the etchants used for metallographic examination.
Oxidation weight change for nickel-2% thoria at various times and temperatures in air at ambient and 8 Torr pressure.
Oxidation weight change for 80 nickel-20 chromium-?,% thoria at various times and temperatures in air at ambient and 8 Torr pressure.
Oxidation weight change for 62 cobalt-18 chromium-20 nickel 2 volume percent thoria at various times and temperatures in air at ambient and 8 Torr pressure.
Oxidation weight change for 50 cobalt-30 chromium-20 nickel 2 volume percent thoria at various times and temperatures in air at ambient and 8 Torr pressure.
Microhardness (DPH) of nickel-2 volume percent thoria alloy after 1000 hours at specified temperatures and pressures.
Microhardness (DPH) of nickel-:! volume percent thoria alloy at various times and temperatures at ambient pressure.
Microhardness (DPH) of nickel-:! volume percent thoria alloy at various times and temperatures at a pressure of 8 Torr.
Page
3
6
7
9
11
13
14
18
18
19
iii
XI
XI1
XI11
XIV
xv
XVI
Microhardness (DPH) of 80 nickel- 20 chromium-:! volume percent thoria alloy after 3000 hours at specified temperatures and pressures.
Microhardness (DPH) of 80 nickel-20 chromium-:! volume percent thoria alloy at various times during exposure to 2000 F at two different pressures.
Microhardness (DPH) of cohalt-20 nickel-18 chromium-2 volume percent thoria alloy after 1000 hours at specified temperatures and pressures.
Microhardness (DPH) of cobalt-20 nickel-18 chromium-2 volume percent thoria alloy at various times during exposure to 2000 F at two different pressures,
Microhardness (DPH) of cobalt-20 nickel-30 chromium-2 volume percent thoria alloy after 1000 hours at specified temperatures and pressures.
Microhardness (DPH) of cobalt-20 nickel-30 chromium-2 volume percent thoria alloy at various times during exposure to 2000 F at two different pressures,
19
20
20
21
21
22
iv
LIST OF FIGURES
Figure No. Title
1. The total metal loss in inches x loo3 (mils) for nickel - 2 volume percent thoria sheet as a function of time and temperature.
2. The total metal loss in inches x (mils) for 80 nickel - 20 chromium - 2 volume percent thoria sheet as a function of time and temperature.
30
4.
5 ,
6.
7.
8.
9 ,
10 0
TD Ni alloy after 1000 hours at 1400 F (760 C) and ambient pressure. 800X
Porous oxide scale (upper portion of photo- graph) on TD Ni after 1000 hours at 2000 F (1093 C) at ambient pressure. 800X
Oxide clusters in TD NiCr after 70 hours at 2000 F (1093 C) and 8 Torr. 800X
Second phase oxide particles (cubic in appear- ance) in the oxide scale of TD NiCr, This is a 1000 hour, 1800 F (982 C) ambient pressure sample. 800X
Plane polarized light micrograph of second phase oxide particles in the oxide scale of TD NiCr exposed for 3000 hours at 2000 F (ambient pressure). 800X
Page
10
12
16
16
16
16
17
Typical structure of TD Co alloy after exposure for any given time or temperature. This sample is 316 hours at 1800 F in air. 800X 17
Rare occurrence of continuous oxide in TD Co alloy. and 8 Torr. 8OOX 17
This sample is 1000 hours at 1800 F
Typical structure of TD Co+ alloy after exposures This sample is 1000 hours at 1800 F and 8 Torr, 800X 17
V
12 a
13
16 e
17 e
18.
19 a
20 a
lA.
Electron micrograph of a replica of a TD Ni sample exposed 1000 hours at 1800 F (982 C) at ambient pressure. 30,OOOX
Electron micrograph of a replica of as- received TD NiCr. 30,000X
Electron micrograph of a replica of a TD NiCr sample after 3000 hours exposure at 2000 F (1093 C) and ambient pressure. 30,OOOX
Electron micrograph of TD Co as-received (replica). 30, OOOX
Electron micrograph of TD Co replicated sample after 1000 hours exposure at 2000 F (1093 C) and 8 Torr. 30,OOOX
Electron micrograph of a replica of TD Co+ after 1000 hours exposure at 2000 F (1093 C) and 8 Torr, 30,OOOX
Cumulative particle size distribution of thoria in TD Ni after 1000 hours at 1800 F (982 C) and ambient pressure,
Cumulative particle size distribution of thoria in TD NiCr after 1000 hour exposure at 2000 F (1093 C) and ambient pressure.
Cumulative particle size distribution of thoria in TD Co after 1000 hours at 2000 F (1093 C) and ambient pressure, together with the as-received distribution.
Cumulative particle size distribution of thoria in TD Co+ after 1000 hours at 2000 F (1093 C) and 8 Torr,
Schematic of oxidation test apparatus consist- ing of mirror image furnace tubes with 5 heat zones in each and one side maintained at 8 Torr by pumping against a controlled leak at He
23
23
24
24
25
25
26
26
27
28
34
vi
1
SUMMARY
Four p o t e n t i a l candidate materials f o r t h i n shee t a i r c r a f t applica-
t ions were t e s t ed i n a i r a t 760 and 8 Torr i n the temperature range 1200
t o 2000 F (650 t o 1093 C).
with t h o r i a , 2 volume percent , and were n icke l base (TD N i ) , 80 n icke l - 20 chromium (TD N i C r ) , coba l t base - 20 n icke l - 18 chromium (TD Co), and
cobal t - 20 n icke l - 30 chromium (TD Cot.).
A l l mater ia l s were d ispers ion strengthened
The exposed specimens were examined by o p t i c a l microscopy, microhard-
ness t e s t i n g , e l ec t ron microscopy, and t h o r i a p a r t i c l e size analysis .
Oxidation rate d a t a were co l lec ted . Microscopy of t h e TD N i C r
suggested t h a t controversy over t he composition of t he oxide l a y e r on
n i cke l - chromium a l l o y s may be t r aceab le t o t h e formation of a chromium
sesqui-oxide (Crq03) l a y e r s a tu ra t ed with n i cke l - chromium s p i n e l
( N i O C r 0 ), t h a t p r e c i p i t a t e s upon slow cool’ing from elevated temperatures.
TD N i C r scaled at 1600 F (870 C) and below while t he cobal t a l l o y s exhibi ted
exce l l en t s ca l ing res i s tance .
2 3
A l l a l l o y s showed evidence of sof ten ing a t t i m e and temperature.
P a r t i c l e s i z e ana lys i s demonstrated t h a t no agglomeration of t h o r i a had
occurred. Softening w a s a t t r i b u t e d t o r e l axa t ion of re ta ined cold work
except f o r t h e low pressure cobal t base samples.
t hese samples w a s a t t r i b u t e d t o s e l e c t i v e chromium l o s s by evaporation.
The g rea t e r sof ten ing of
Because of severe oxidat ion and sca l ing TD N i is not recommended f o r
high a l t i t u d e , high Mach number se rv i ceo Pre-oxidized TD N i C r is accept-
a b l e a t a l l temperatures t o 2000 F (1093 C) f o r a t least 3000 hours and
both TD cobal t a l l oys t o a t l e a s t 1000 hours.
2
INTRODUCTION
There e x i s t s a need f o r t h i n gage oxidation r e s i s t a n t supera l loys
for t h e s k i n of high-flying high Mach number a i r c r a f t of t h e f u t u r e as
w e l l as t h i n gage a f t e rbu rne r s h e l l s , furnace walls and t h e l i k e .
The p r i n c i p a l ob jec t ive of t h i s s tudy was t o determine t h e s u i t a b i l i t y
of d i spers ion strengthened a l l o y s f o r t h i s kind of serv ice .
t u r e range of i n t e r e s t w a s 1200 t o 2000 F and t h e a i r pressure v a r i a t i o n
8 Torr t o ambient.
The tempera-
P rope r t i e s t o be examined were r e s i s t a n c e t o oxida t ion and r e s i s t a n c e
t o agglomeration of t h e dispersoid.
Secondary ob jec t ives were t o examine t h e oxida t ion behavior and t o
expla in t h e mechanism of agglomeration, where i t occurred.
It was des i red t o examine a l l candidate a l l o y s t h a t exhib i ted reason-
able oxida t ion r e s i s t ance .
ment were nickel-chromium o r nickel-cobalt-chromium ana lys i s , wi th t h o r i a
All a v a i l a b l e alloys o r those under develop-
as t h e p r i n c i p a l d i spersq id .
Oxidation behavior of t hese a l l o y s w a s of i n t e r e s t i n t h r e e respects.
F i r s t , the e f f e c t of long-time exposure on t h i n shee t w a s uncertain; and
second, t h e e f f e c t a t 8 Torr needed t o be examined. Third, t h e l i t e r a t u r e
concerning t h e oxida t ion behavior of nickel-chromium a l l o y s was not a t
a l l i n agreement on t h e oxida t ion mechanism involved,
Before examining nickel-chromium
n i c k e l is of i n t e r e s t . This t o p i c i s
oxidation of n i c k e l forms an adherent
t he oxidation behavior of pure
covered i n d e t a i l by IIauffe e The 1
laye r of N i O , which is a p type s e m i -
3
T, OF
1200
1380
conductor. The rate con t ro l l i ng mechanism i s by N i d i f fus ion through the
T, O C
650
7 50
oxide layer .
is 34 kcal/mole and AS is -17.3 cal/mole - ' C .
The AH f o r t h e process between 750 and 1650 F (400 and 909 C)
Hauffe r e f e r s t o work by Wagner and Zirnens and P f e i f f e r and Hauffe,
2 4 k", gm /cm - see.
2.32 10-l~
1.46 x
9.49 10-l~
1.81 x
t h a t shows a small amount of chromium increases t h e oxidation rate as would
be expected s i n c e t h e Cr+3 increases the e l e c t r o n i c conductivity of t h e N i O .
AH AS AF
38 , 150 -15 . 7 52,650
38,150 1 54,750
38,150 55,900
38,150 55,450
Hauffe published d a t a from two sources showing t h a t more than 6 percent
1560
1610
chromium i n n i c k e l reduces t h e acce le ra t ing e f f e c t of smaller amounts of
850
875
chrumium, a t least a t 1800 F (982 C). Beyond t h a t point n i c k e l chromium
a l l o y s are more oxida t ion r e s i s t a n t than n icke l .
Table I is a compilation of da t a on 80 N i - 20 C r a l l o y by Gulbransen
2 and Andrew e
Table I
Oxidation rates of an 80 N i - 20 C r a l loy .
Note: k" is t h e pa rabo l i c rate constant i n t h e equation:
(where - t is t i m e , and - E is scale thickness),
These rates compare t o about 6 x a t 1607 F (875 C) f o r u l t r a pure
1 3 n i c k e l e Other rate d a t a can be found i n Gulbransen and Andrew , Wood and
4 5 6 Hodgkiess , Ignatov and Shamgunova and Jones and Westerman t h e lat ter
r e fe rence concerned wi th t h e oxidation rate of a n i c k e l a l l o y containing
E* = k"t
4
7 8 2% dispersed thor ia . Wlcox and Clauer and Cole, et a l . , have made
f u r t h e r observations of nickel-chromium dispers ion strengthened with tho r i a ,
These au thors agree on a t least t h r e e points:
1. Alloys of 80 n i c k e l - 20 chromium are super ior t o pure
n i cke l , i n s o f a r as oxidation rate is concerned.
Minor impur i t i e s o r small a l l o y add i t ions have a marked 2.
e f f e c t on rate.
3. A t reasonable rimes (10 hrs . ) . and temperatures (above 1380 F
(750 C ) ) , t h e p r o t e c t i v e oxide l a y e r is not Ni0,
Beyond these po in t s , however, t h e a c t u a l mechanism of oxida t ion is
much i n question.
1 Hauffe d iscusses t h e oxidation mechanism of n i c k e l chromium a l l o y s
i n d e t a i l , p. 170 - 193, and concludes t h a t because of Cr03 v o l a t i z a t i o n ,
N i C r 2 O 4 s p i n e l is favored a t 2012 F (1100 C) and above and t h a t t h e forma-
t i o n of C r 0 is improbable, Ignatov and Shamgunova r epor t a - formation below 1292 F (700 C) and double l a y e r f i lms above 1472 F (800 C )
5 Cr203 2 3
with t h e o u t e r l a y e r cons i s t ing of NiCr204.
l a y e r s ind ica ted a q u i t e cons i s t en t s p i n e l with a l a t t i ce parameter of
8,31 A,
X-ray examination of these
0
Unfortunately, t h e chemical a n a l y s i s of t h e specimen a l l o y is not
completely given i n regard t o Mn, Fe, etc, content. These elements are
3 very important i n regard t o s p i n e l formation.
r epor t only small t r a c e s of NiCr204 s p i n e l up t o and including 2012 F
(1100 C) i n N i - C r h e a t e r a l l o y s but express d i s s a t i s f a c t i o n with t h e
X-ray d i f f r a c t i o n of oxide sca l e s , Burks and Rickert examined n i c k e l
Gulbransen and Andrew
9
a l l o y s containing 30, 60, and 80 percent chromium, I n a l l cases only
C r 0 was found, They f u r t h e r c a l c u l a t e t h a t chromium content i n t h e a l l o y 2 3
5
must f a l l t o about los7 before N i C r 2 0 4 becomes the s t a b l e phase a t 2012 F
(1100 C ) ,
n i c k e l - chromium a l l o y s wi th g r e a t e r than 14.6% chromium.
high p u r i t y a l l o y s tock , and base t h e i r conclusions e n t i r e l y on micro-
Wood and Hodgkiess4 conclude t h a t only C r 0 scale occurs on 2 3
They worked with
probe ana lys i s ,
of 49 n i cke l , 22 chromium.
They mention a re-occurring l a y e r i n a s t r a t i f i e d scale
This amounts t o a 2:l atom r a t i o of N i t o C r ,
which could w e l l be a 3:l mixture of N i O and N i C r 2 0 4 .
d id not attempt any X-ray d i f f r a c t i o n experiments.
Unfortunately, they
Previous work concerning t h e s t a b i l i t y of d i spers ion strengthened
a l l o y s has genera l ly been d i r ec t ed toward s h o r t t i m e s a t high temperatures.
Worn and Marton" s tudied t h o r i a d i spersed i n n i c k e l and concluded
t h a t i ron , chromium, and s u l f u r contamination favored coarsening of t h e
dispersoid. Weeton and Quatinetz" demonstrated t h a t s u l f u r contamination
caused d e f i n i t e agglomeration i n t h o r i a dispersed n icke l .
t h a t hydrogen cleaning has a favorable e f f e c t on d isperso id s t a b i l i t y .
They concluded
Chang12 examined an 80 n i c k e l - 20 chromium - t h o r i a a l l o y containing
about 5% d i f fused aluminum held a t 2300 P (1260 C) f o r 200 hours. No
agglomeration was de tec ted , Raymond and Neumann13 s tudied t h e high tempera-
t u r e s t a b i l i t y of two 80 n i c k e l - 20 chromium - 2 volume percent t h o r i a
a l l o y s from two d i f f e r e n t sources. Softening occurred i n one a l l o y a t
temperatures above 1800 F (982 C). This a l l o y had a l a r g e r average p a r t i c l e
s i z e as fab r i ca t ed , and suf fered an apparent growth i n average p a r t i c l e s i z e
from 820 A t o 885 A. 0 0
The second a l l o y vas a product produced by a DuPont
process and w a s apparently s t a b l e f o r 1 hour exposures a t a l l temperatures
i n t h e study, up t o 2400 F (1315 C).
6
-- Number Alloy Composition Code
- _II_I- 1 Nickel + 2 volume percent t h o r i a (TD N i )
2 80 n i c k e l - 20 chromium - 2% t h o r i a
3 Cobalt - 20 n i cke l - 18 chromium - 2% t h o r i a (TD Co)
(TD NiCr)
EXPERIMENTAL PROCEDURES
Cobalt - 20 n i c k e l - 30 chromium - 2% t h o r i a 4
Four d i f f e r e n t materials were examined during t h i s study, Table 11.
(TD Co+) 1 !
Table 11
Dispersion strengthened a l l o y s examined in t h e present study.
A l l t h e a l l o y s are products of t h e Fans tee l 14etallurgical Co. w i t h
a l l o y s 1 and 2 furnished by t h a t company and a l l o y s 3 and 4 furnished from
an A i r Force-Pratt tlhftney program.
t h e s h o r t form code l i s t e d i n Table 11.
*
These a l l o y s w i l l be r e fe r r ed t o by
TD Niwas used as 13-15 m i l shee t ,
TD N i C r as 10 m i l shee t , and because of shortage of a l l o y the cobal t base
a l loys were used as small pieces pr imar i ly 30 t o 40 m i l s t h i ck ,
The TD Ni and TD N i C r were c u t i n t o coupons about 1 square inch. A l l
samples were cleaned, weighed, and measured. The a l l o y samples were then
exposed a t 1200, 1400, 1600, 1800, and 2000 F a t 8 Torr and ambient pressure
in a twin gradien t furnace, which is described f n d e t a i l i n t h e Appendix.
The furnace w a s cooled slowly a t i n t e r v a l s and samples removed. The
samples were v i s u a l l y examined, weighed, and a s e c t i o n c u t f o r microscopy.
Pieces c u t f o r examination were mounted i n a s p e c i a l j i g t h a t provided a
4:f t ape r s e c t i o n i n order t o estimate oxida t ion pene t r a t ion and provide
7
ITD Ni ' I 100 nickel - 2% thoria
more area for viewing,
The etchants used for the 4 alloys are listed in Table 111,
3 HN03: 2 Acetic Acid
TD NiCr 1 80 nickel - 20 chromium - 22 thoria ' 3 HC1: 1.5 HN03: 10 H20 + CuC1; I I I
TD Co Cobalt - 20 nickel - 18 chromium - ' Marbles reagent i i 2 x thoria I i Cobalt - 20 nickel - 30 chromium -
2% thoria Marbles reagent
i I . -. . _ _ .. . . .. _. . ... .. . ......__ .A ... ...... ... . - - ..---
Table I11
Four alloys examined in the present study and the etchants used
for metallographic examination.
Microhardness readings were taken using a Bergsman tester, vickers
diamond, and a 100 gm. load.
Particle size determination and distribution was carried out using
replica electron microscope techniques, a Ziess TGZ-3 counter, and micro-
graphs at 30,000 X.
8
RESULTS
Three kinds of d a t a were evaluated concerning oxidation behavior;
t hese were weight change, metal loss, and sca l ing behavior.
The TD N i exhib i ted progressive weight gain with increas ing temperature
and time a t ambient pressure but showed weight loss a t 8 Torr f o r a few
specimens. The loss i n specimen These d a t a are presented i n Table IV.
thickness is given i n f i g u r e 1.
t h e earlier oxida t ion study of llanning e t al .I4
temperatures i n t h e s tudy with l a r g e areas of s c a l i n g a t 1200 - 1600 F
(650 - 870 C).
Note t h a t t h e va lues f o r a i r agree with
TD N i s c a l e s a t a l l
TD N i C r proved more r e s i s t a n t t o oxida t ion and sca l ing than TD N i .
Weight changes were small, with both ga ins and l o s s e s recorded, Table V.
Since t h i s a l l o y was received e a r l y i n t h e piogram, weight changes are
reported t o 3000 hours. The loss i n specimen thickness is shown i n
f i g u r e 2. This a l l o y is q u i t e supe r io r i n r e s i s t a n c e t o sca l ing a t high
temperatures; however, samples scale a t 1200 - 1400 F (650 - 760 C) upon
cooling and t h e 1600 F (870 C) samples show evidence of f l ak ing during
exposure . I n oxida t ion behavior t h e TD Co and TD Co+ a l l o y s are indis t inguish-
able. These a l l o y s exhib i ted less metal loss than t h e TD N i C r and have
exce l l en t scale r e s i s t a n c e a t a l l temperatures. Desp i t e good scale adher-
ence both a l l o y s c o n s i s t e n t l y l o s t weight, Tables V I and V I I , Because of
t h e s c a r c i t y of material, these a l l o y s were t e s t e d only a t 1600, 1800
and 2000 F (870, 982 and 1093 C).
. The s t r u c t u r a l s t a b i l i t y of t h e a l l o y s was inves t iga ted through o p t i c a l
9
Temperature O F
1200
1400
1600
1800
2000
1200
1400
1600
1800
2000
Oxidation weight change for nickel-2% thoria at various
ambient and 8 Torr pressure. times and temperatures in air at
2 -3 Wt. Change, gms/cm x~ 10-
100
+ 0.52
+ 1.72
+ 5,15
+ 8.42
+ 18.25
+ 0.27
+ 1.12
+ 2.26
+ 7.03
+ 13.60
at time, hrs.
316
+ 1,51
+ 2.39
+ 4.65
+ 13.20 + 25.40
- 0.12 ’
+ 1.04
+ 2.96
- 17,33 + 12.10
1000
+ 0.43
+ 2.48
+ 7.68
+ 17.80 + 56.60
+ 0.49
+ 1.55
+ 5.05
- 4.61
$. 2.12
Ambient
Air
8 Torr
Table IV
L
1
IO
Figure 1. 2 volume percent thoria sheet as a function of time and temperature in a ir .
Two data points are included from Manning et by converting their weight gain data into metal thickness loss. 1800 P in a ir are the same as 2000 F, 8 Torr within experimental error.
The tota l metal l o s s i n inches x l f 3 (mils) for nickel -
The results given for
11
+ 0.24
+ 0.32
Temperature OF
8 Torr
1200
1400
1600
1800
2000
1200
1400
1600
1800
2000
Oxidation weight change for 80 nickel-20 chromium-2% thoria at various
times and temperatures in air at ambient and 8 Torr pressureo
2 Wto Change, gms/cm x lom3
70
- -
+ 5.45
+ 1.25
- - -
- 1.00
- 17.80
1 at time, hrs.
316
- 3.31 - 3.39
0
+ 6.85 + 4.12
- 1.60 + 1.69 - 0.15 - 1.58 + 1.63
1000 1 3000 1 + 0.18 I - 5.27
+ 2.29 + 0.12 - 3.04
-t 1.85
-t 0.16
- 0.49 I - 3*88 I - 1.00
-,0.86
Ambient
Air
___?__ 4- 0.02
I - 0.49 I + 9.18 - 1.02 - 7.20 + 1.40 I - 9.93 I
Table V
loo
T I M E ,HOURS
Figure 2. The t o t a l metal l o s s i n inches x (mils) f o r 80 n icke l - 20 chromium - 2 volume percent t h o r i a shee t as a func t ion of time and temperature i n air.
1000
The r e s u l t s f o r ambient pressure are t h e same as those f o r 8 Torr wi th in experimental e r r o r , magnitude from those i n f i g u r e 1 f o r unalloyed n i cke l - thor ia .
Note t h a t these r e s u l t s d i f f e r by an order of
13
Temper a ture OF
1600
1800
2000
__
1600
1800
2000
i-
Oxidation weight change for 62 cobalt-18 chromium-20 nickel 2 volume percent thoria a t various times and temperatures i n a i r at
ambient and 8 Torr pressure.
2 W t . Change, gms/cm x f
a t t i m e , hrs.
100 316 1000
- 4.25 * 1.55 + 1.48 __e
Ambient
A i r - 0.40 - 1.74 - i s a
I
4- 2.22 * 2.52 - 0.79 -_
- 1.98 - 1.50 - 0.52 - 4.70 - 2.61 - 1.35 8 Torr
- 3.21 - 3.96 - 3,20 9
,Table V I
14
Temperature O F
1600
1800
2000
1600
1800
2000
Oxidation weight change for 50 cobalt-30 chromium-20 nickel
times and temperatures in air at ambient and 8 Torr pressure.
2 volume percent thoria at various
~
Wt. Change, gms/cm2 x
at time, hrs.
100
- 2.20 - 2-80 - 1.27
- 1.66 - 3.66 - 1.87
316
- 3.02 - 3.56 - 2.73
- 1.87 - 4.11 - 4.58
1000
- 0.74 - 1.17 - 2.00
- 1.91 - 1.62 - 1.96
Ambient
Air
8 Torr
Table VI1
15
microscopy, microhardness t e s t i n g , e l e c t r o n microscopy, and subsequent
p a r t i c l e s i z e ana lys i s ,
The TD N i and TD N i C r a l l o y s were charac te r ized by t h e formation of
hollow sphere groupings of t h e l a r g e inc lus ion- l ike oxides v i s i b l e i n t h e
o p t i c a l microscope.
hours a t 1400 F (760 C) a t ambient pressure i s shown i n f i g u r e 3 a t 800X0
The porous oxide formed by TD N i a t 2000 F (1093 C) f o r t h e same t i m e
and p res su re is shown i n f i g u r e 4. The sphe r i ca l ( c i r c u l a r ) oxide group-
ing is shown i n f i g u r e 5 which is a micrograph of TD N i C r a f t e r 70 hours
at 2000 F (1093 C) and 8 Torr. The arrows poin t t o t h e oxide array.
A r ep resen ta t ive area of t h e TD N i a l l o y a f t e r 1000
Small second-phase oxide p a r t i c l e s i n t h e oxide scale of TD N i C r are
shown c i r c l e d i n f i g u r e 6 i n ordinary l i g h t and i n f i g u r e 7 i n plane polar-
ized l i g h t (crossed f i l t e r s ) . The t y p i c a l s t r u c t u r e of TD Co a l l o y a t 800X
is shown i n f i g u r e 8. One of t h e rare occurrences of continuous oxide i n
t h e cobal t base a l l o y s is shown i n f i g u r e 9. The t y p i c a l s t r u c t u r e of t h e
TD Co+ a l l o y is shown i n f i g u r e 10 a t 8OOX.
Microhardness of TD N i samples a f t e r 1000 hours exposure i s given i n
Table V I I I .
is given i n Table I X and f o r 8 Torr samples i n Table X,
Microhardness v a r i a t i o n of ambient pressure samples with t i m e
The microhardness d a t a f o r TD N i C r is given i n Table X I f o r 3000 hour
exposure, and i n Table XI1 as a func t ion of t i m e . Microhardness readings
for TD Co and TD Co+ show 8 s i g n i f i c a n t drop with t i m e f o r 8 Torr exposureo
These d a t a are given i n Tables XI11 and X I V f o r TI3 Co and Tables XV
and XVI f o r TD Co+,
Two-stage e l e c t r o n microscope r e p l i c a s - o f specimens from each a l l o y
group were examined,
Figure 3. TD N i a l l o y a f t e r 1000 hours a t 1400 F (760 C ) and ambient pressure . 800X
F i g u r e 5, Ox ide c l u s t e r s i n Tr? NrLCr a f t e r 70 1:stnrs at 2060 F (1033 C) and R T o r r , ROOX
Figure 4. Porous oxide s c a l e (upper por t ion of photograph) on TD N i a f te r 1 0 hours a t 2000 F (1093 C) a t ambient pressure. 800X
F i g u r e 6, Second phase nxirle p a r t i c l e s (cubic i n appearance) in the oxide s c a l e of TD M i C r , Tlrio I s a 1 O O O Ilottrp 1800 F (982 C) amhient pe-esst1-e.e sample, 800X
Figure 7, Plane po la r i zed l i g h t micrograph of second phase oxide p a r t i c l e s i n the ox ide s c a l e of TD N i C r exposed f o r 3000 hours a t 2000 F (ambient p r e s s u r e ) , 800X
Figure 9, Rare occurrence of continuous ox ide i n TD Co a l l o y , This sample Is 1000 hours at f800 F and 8 Tar r , 800X
Figure 8. Typical s t r u c t u r e of TD Co a l l o y a f t e r exposure f o r any given time o r temperature. This sample i s 316 hours a t 1800 I? i n a i r . 800X
F i g u r e LC. Typlesl structure of TD Go+ a21ov a f t e r exposure, T h i s sample i s 1000 hours at 1800 F and 8 Torr, 800X
..,
18
_______ _ _ - - - - - - Exposure Temperature OF
I As !
i ' Received j 1200 1400 1600 1800 2000
~- .. I__ __ __________ -. -. - .. _ _ . . . ___-- I Ambient I
I
202 ! 199 200 201 180 Oxidized P res su re f
201 192 192 185 Oxidized 8 Torr 1 202 I I i i i
_I__^_,I-__x____
Microhardness (DPH) of nickel- 2 volume percent t h o r i a a l l o y a f t e r
1000 hours a t spec i f i ed temperatures and pressures.
Table VI11
Temperature O F
1800
2000
Time, h rs .
100 316 1000
190
193
189
189
180
Oxidized
As received hardness, 202
Microhardness (DPH) of nickel- 2 volume percent t h o r i a a l l o y a t var ious times and temperatures
at ambient pressure .
Table I X
19
.. ,
Temperature O F
i i
Time, hrs . ~ I _ _ _ - !-
I I i 100 316 1000
1600
1800 i i
! 2000
202 202 I
201
201
198
193
192
185
Oxidized I ! I^ _ _ _ . . . _ _. .. .. . ... . .... .
As received hardness, 202 I
Microhardness (DPH) of nickel- 2 volume percent thoria a l l c y at various times and temperatures
at a pressure of 3 Torr.
Table X
As Received
I , --- -- _. -___________
Ambient
Pressure I Air i 320
Exposure Temperature OF
1200 1400 1600 1800 2000 -- _- - I _-_ -
297 297 304 298 297
8 Torr 320 299 304 297
Microhardness (DPH) of 80 nickel- 20 chromium-2 volume percent thoria alloy
after 3000 hours at specified temperatures and pressures,
Table XI
20
Ambient Air
Pressure
t 8 Torr
A s Received
320
320
I
20
Time, hrs.
100 316 1000 3000 ~ - - - - -^_- _I---
-
302 303 303 297
302 306 302 2 98
Microhardness (DPH) of 80 nickel- chromium-2 volume percent thoria alloy at various times during exposure to 2000 F at two different pressures.
A s Received
_-_ -I;-- Ambient
8 Torr I 383
Table XI1
D
Exposure Temperature O F
1600 1800 2000
37 6 370 37 6
37 0 354 357
Microhardness (DPH) of cobalt-20 nickel-18 chromium-
2 volume percent thoria alloy after 1000 hours at specified
temperatures and pressures a
Table XI11
Microhardness (DPH) of cobalt-20 nickel-18 chromium-
2 volume percent thoria a l l o y a t various times
during exposure to 2000 F a t two different pressures.
-- - I-___ - - - ____ _I
Table XIV
Time, hrs . Received
As . I 100 316 1000 -__--
AS Received
Ambient
Pres sure I Air 390
8 Torr 390
I I I I
Exposure Temperature O F
1600 1800 2000 - I__--
366 363 366
351 351 354 I
21
Microhardness (DPH) of cobalt-20 nickel-30 chromium-
1000 hou'rs at. specified temperatures and pressures.
2 volume percent thoria alloy after
Table XV
22
Time, h r s . 100 316 1000
A s Received
I
Ambient
Pressure A i r . 390
1 ---
387 370 366
-__I--.---- -_I
Elec t ron micrographs of each all-oy were taken throughout t h e program.
Figure 11 is a micrograph of TD N i a f t e r 1000 hours exposure a t 1809 F
(982 C) a t ambient pressure. Figure 12 i s a micrograph of TD N i C r as
received and Figure 13 i s t h e same material a f t e r 3000 hours exposure a t
20013 F (1093 C) a t ambient pressure.
Figure 1 4 i s an e l ec t ron micrograph of TD Co as received and f i g u r e
15 is a specimen of TD Go a f t e r 1000 hours exposure a t 2000 F (1093 C ) a t
8 Torr. Figure 1 6 shows t h e r ep l i ca t ed s t r u c t u r e of TD Co+ a l l o y a f t e r
1000 hours a t 2000 F (1093 C) a t 8 Torr.
The p a r t i c l e s i z e d i s t r i b u t i o n f o r TD N i a f t e r lO(fT) hours a t 1800 F
(982 C) and ambient pressure i s shown i n f i g u r e 17, The p a r t i c l e s i z e
d i s t r i b u t i o n f o r TD N i C r is p l o t t e d f o r a 1000 hour, 2000 F (1093 C),
ambient pressure sample i n f i g u r e 18. Data from Chang12 f o r TD NiCr shee t
Figure 11. Electron micrograph of a replica of a TD Hi sample exposed 1000 hours at 1800 F (982 C) at ambient pressure.
Figure 12. Electron micrograph of a repl ica af as-received TD NICr.
Figure 13. Electron micrograph of a replica of a TD Nice sample after 3000 hours exposure at 2000 F (1093 C) and ambient pressure. 30, OOOX
B
Figure 1 4 . Electron micrograph of TD Co as-received (replica). 30, OOOX
Figure 15. Electron micrograph of TD Co replicated sample after 1000 hours exposure at 2000 F (1093 C) and 8 Torr. 30, OOOX
P
Figilre 16. Electron micrograph of a rep l ica of TD Co+ after 1OO0 hours exposure a t 2000 F (1093 C) and 8 Torr. 30.000X
26
THORIA S I Z E , ANGSTROMS x io2 Figure 17. Cumulative p a r t i c l e s i z e d i s t r i b u t i o n of t h o r i a i n TD Ni a f t e r 1000 hours a t 1800 F (982 C) and ambient pressure.
2 3 4 - 5 6 7 8
THORIA SIZE. ANGSTROMS x 10' Figure 18, 'ED N i C r a f t e r 1000 hour exposure a t 2000 F (1093 C) and ambient pressureo for t h e same a l l o y ana lys i s and manufacturer adapted from ref , 12 is included.
Cumulative p a r t i c l e s i z e d i s t r i b u t i o n of t h o r i a i n
A p a r t i c l e s i z e d i s t r i b u t i o n for as-received material
27
from t h e same manufacturer is a l s o given i n f i g u r e 18.
The p a r t i c l e s i z e d i s t r i b u t i o n f o r TD Co af ter 1000 hours a t 2000 F
(1093 C) and ambient pressure i s shown i n f i g u r e 19.
of t h e p a r t i c l e s i z e d i s t r i b u t i o n f o r TD Co+ a f t e r 1000 hours a t 2000 F
(1093 C) and 8 Torr.
Figure 20 i s a p l o t
The d i s t r i b u t i o n curves are p l o t t e d on t h e b a s i s of equal volume
(area) s i n c e the observed number of t h e smallest particles depends g r e a t l y
on response t o etching.
2 3 4 5 6 7 a THORIA SIZE, ANGSTROMS x i o 2
Figure 19. TD Co a f t e r 1000 hours a t 2000 F (1093 C) and ambient pressure , toge ther wi th t h e as-received d i s t r i b u t i o n .
Cumulative p a r t i c l e s i z e d i s t r i b u t i o n of t h o r i a i n
28
3 0
Figure 20. TD Cot. after 1000 hours at 2000 F (1093 C) and 8 Torr.
Cumulative particle s i z e distribution of thoria in
DISCUSSION
29
The oxida t ion rate d a t a were about as expected. The TD N i oxidized
near ly t e n times faster than t h e TD N i C r and .TD coba l t a l loys . The
TD N i is hampered by s c a l i n g a t every cool-down from a l l temperatures.
Several f a c t s emerged from t h e oxidation study i n regard t o t h e
alloyed TD materials. The TD N i C r exhib i ted about t h e same thickness of
oxide and oxide co lo ra t ion f o r 8 Torr specimens a t 200 F above those f o r
ambient pressure.
(650 and 760 C) a t ambient and up t o 1600 F (870 C) a t 8 Torr. This f a c t o r
leads t o t h e recommendation given later concerning preoxidation of TD N i C r
a t 1600 F (870 C) minimum. The TD coba l t a l l o y s were as oxidation resist-
The TD N i C r specimens flaked a t 1200 and 1400 F
ant as the TD N i C r a l l oy , bu t d i d not scale o r f l a k e a t any temperature
o r pressure condition.
The type of oxide scale
20 chromium p r i n c i p a l l y ) has . J c
P
present on nickel-chromium a l l o y s (80 nickel-
been repor ted t o be pure C r 0 4,9 o r 2 3
N i C r 0 Cr203 mixtures3’3.
exhib i ted two phase oxide scale of t h e type shown i n f i g u r e s 6 and 7.
A g r e a t many samples of TD N i C r i n t h i s study 2 4-
The
second phase appears t o be a p r e c i p i t a t e s i n c e i t is composed of small
cubic p a r t i c l e s i n t h e hexagonal Cr203 matrix.
Cr20j is sa tu ra t ed with NiCr204 s p i n e l a t temperature and t h a t t h e s p i n e l
p r e c i p i t a t e s upon cooling.
This suggests t h a t t h e
This hypothesis would suggest t h a t an explana-
t i o n f o r t h e varying r e s u l t s of previous inves t iga to r s would l i e i n examin-
ing t h e cooling rates employed i n t h e i r s tud ies .
favor a Cr203 so lu t ion and a slow cool (as employed i n t h i s study) would
favor t h e p r e c i p i t a t e .
A rap id cooling would
30
Since t h e TD coba l t a l l o y s d id not scale, t h e weight l o s s f igu res
should be explained i n some way. Two c h a r a c t e r i s t i c s of t he d a t a can he
in t e rp re t ed t o expla in t h e weight loss . F i r s t , i t should he noted t h a t
t h e 18 percent chromium a l l o y , TD Co Table V I , d i d not exh ib i t as l a r g e
a weight l o s s as t h e 30 percent a l l o y , TD Co+ Table VII. Second, t h e
weight l o s s f o r both a l l o y s is more severe a t 8 Torr than a t ambient
pressure.
chromium because of t h e high vapor pressure of C r O
These two phenomena are i n t e r p r e t e d as a p r e f e r e n t i a l loss of
3" The s t a b i l i t y of each of t h e sub jec t a l l o y s is an important considera-
t i o n beyond t h e i r r e s i s t a n c e t o material l o s s through oxidation.
A l l a l l o y s su f fe red a small l o s s i n hardness i n the f i r s t 100 hours
of exposure.
type.
respec t ive ly .
increase i n p a r t i c l e s i z e t o accompany t h e drop i n hardness. Hardness
va lues and p a r t i c l e s i z e ana lys i s f o r TD N i C r ind ica ted no s i g n i f i c a n t
change f o r 3000 hours a t a l l test conditions.
The long t i m e hardness d a t a were d i f f e r e n t f o r each a l l o y
TD N i exhib i ted a drop from 190 DPH t o 180 DPH a t 100 and 1000 hours
P a r t i c l e s i z e a n a l y s i s (figure, 16) d id not c l e a r l y show any
Both coba l t a l l o y s suf fered a drop i n hardness during exposure a t
8 Torr f o r a l l temperatures.
microscope and p a r t i c l e ana lys i s ind ica ted t h a t no change i n t h e d ispers ion
had occurred up t o t h e 1000 hour tes t mark. This behavior can be explained
i n the same way as t h e weight l o s s during t e s t i n g , i.e., as a drop i n hard-
nes s due t o chromium los s .
ness va lues are e s s e n t i a l l y t h e same f o r 1600, 1800 and 2000 F (870, 982
and 1093 C) TD Co+ samples af ter 1000 hours ,at 8 Torr, Table XV.
Examination of t h e a l l o y s i n t h e e l e c t r o n
The same phenomena a l s o expla ins why t h e hard-
The p a r t i c l e s i z e ana lys i s showed t h a t f o r t hese manufactured a l l o y s
t h e o r i g i n a l p a r t i c l e s are l a r g e r as t h e a l l o y becomes more complex, with
31
the TD Co exhibiting the largest particle size over the sections examined
in this study.
to instability, however, as all the alloys demonstrated excellent agglom-
eration resistance. For one example, the TD Co, figure 19, appeared to
have a finer particle distribution after exposure, due probably to sample
differences from area to area.
The variation in starting particle size did not contribute
CONCLUSIONS AND RECOMMENDATIONS
Material loss rates during oxidation are plotted for TD Ni and
TD NiCr.
requirements.
severity of cool-down might well affect loss rates adversely.
ing conclusions are drawn from the present study.
These data can be used as base line estimates for light service
It should be borne in mind that temperature cycling and
The follow-
1. TD Ni, TD Co, and TD Cof show RO thoria agglomeration
after 1000 hours of exposure between 1200 F (650 C) and
2000 F (1093 C) at ambient or 8 Torr pressures.
TD NiCr exhibits no measurable agglomeration after 3000
hours of exposure between 1200 F (650 C) and 2000 F
(1093 C) at ambient or 8 Torr pressure.
Transport and spheroidization of unreduced chromium oxides
*
2.
3,
occur in TD alloys containing chromium at times under 1000
hours with a resulting structure that is more favorable
mechanically.
The four alloys examined soften at higher temperatures
in a few hours by 10 to 40 points IDPH).
4,
32
5 , TI) N i softens slightly with increased time at temperature
(10 to 20 points DPII), while TD NiCr is unaffected and
TD Co and TD Co+ soften significantly at 8 Torr.
6. Two phase oxidation wzs observed in TD NiCr and attributed
t o precipitation of Ni0.Cr 0 spinel during specimen cooling. 2 3 It is recommended that TD N i be considered suspect in any applica-
tion that involves cyclic temperature exposure. TD NiCr shows excellent
scaling resistance only at elevated temperatures and should be pre-
oxidized above 1600 F (870 C) if it is to be used under cyclic conditions
at lower temperatures. Cobalt-nickel-chromium allovs appear sensitive
t o low pressure exposure and should be employed with due care in that
kind of service,
Further research is recommended in order to affirm the loss of
chromium at low pressure that has been hypothesized in this report.
work might well be done by electron microprobe analysis.
This
Because these alloys can only be economically applied as long time
structures, exposure times should be extended to 10,000 hours or longer.
Finally, other dispersion strengthened alloys should be investigated
over the times and temperatures used i n this study t o determine their
respective resistances to agglomeration and to explain agglomeration
behavior if it does occur.
33
APPENDIX
The design of t h e furnace assembly used f o r environmental t e s t i n g
was somewhat unique and i s the re fo re described i n d e t a i l i n t h i s
appendix.
The apparatus w a s b a s i c a l l y two gradien t furnaces arranged i n a
mir ror image synnnetry. The furnace chambers were impervious alumina,
closed a t one end and capped wi th stainless steel a t t h e o ther end as
shown i n f i g u r e 1A.
construction. One s i d e was sea led with s i l i c o n e a t t h e s t a i n l e s s cap
and a small hole provided i n t h e inconel thermocouple pro tec t ion tube
Both s i d e s were i d e n t i c a l i n regard t o thermal
(at H i n f i g u r e 1A).
A t h r o t t l e d mechanical pump w a s used t o p u l l a constant 8 Torr B
vacuum aga ins t t h e leak.
Each zone furnace w a s i nd iv idua l ly set using an autotransformer.
The 1200 F (650 C) zones were cont inua l ly monitored using a two pen Varian
recorder. Both chambers were t raversed with a thermocouple from t i m e t o
t i m e and t h e temperature versus d i s t ance p lo t t ed t o compare t o t h e des i red
gradient.
f o r a complete run wi th in 515 F and t o 25 F of t h e set poin t over t h e
Tun e
It proved poss ib l e t o hold t h e grad ien t t o t h e des i red l e v e l
. * * . 9 . .
9 . .
P Q . -
c - a
A Ceramic tube 2 1/8 I D x 36" 8 Stainless cap
C Ceramic "dee" tube D Inconel T/C protection tube
Figure 1A. 5 heat zones i n each and one s i d e maintained a t 8 Torr by pumping against a controlled leak a t H.
Schematic of oxidation tes t apparatus consisting of mirror image furnace tubes with
w
35
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1.
2.
3,
4.
5 0
6.
r
7.
8.
9.
10 . 110
12 0
13 .
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Worn, D. R. and Marton, S o F., Power Metallurgy, Ed. by W. Lesynski, Interscience, New York, 1961, p. 309.
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